The present invention generally relates to an image transfer blanket construction to be used principally in offset printing systems, and more particularly to an image transfer blanket which is flexible and yet which has a non-extensible backing.
In offset lithography, a rotary cylinder is covered with a printing plate which normally has a positive image area which is receptive to oil-based inks and repellent to water and a background area where the opposite is true. The printing plate is rotated so that its surface contacts a second cylinder covered with a rubber-surfaced ink-receptive printing blanket (sometimes also called a printer""s blanket, or broadly an image transfer blanket). One common type of printing blanket is one which is manufactured as a flat, fabric reinforced sheet having an elastomeric, ink-receptive surface. The ink present on the image surface of the printing plate transfers, or offsets, to the surface of the blanket. Paper or other sheet stock to be printed is then passed between the blanket-covered cylinder and a rigid back-up cylinder to transfer the image from the surface of the blanket to the paper.
During the step in which the inked image is transferred from the plate to the blanket and the step where the image is transferred from the printing blanket to the paper, it is important to have intimate contact between the two contacting surfaces. An exact amount of interference pressure is required so that the blanket contacts and removes ink from the printing plate and transfers the inked image to a proper depth into the paper. This is ordinarily achieved by positioning the blanket-covered cylinder and the supporting cylinder it contacts so that there is a fixed interference between the two and so that the blanket is compressed throughout the run to a fixed depth, typically approximately 0.05 to 0.10 mm (0.002 to 0.004 inches). It is important that this compression be maintained uniformly over the entire surface of the blanket.
Within the current state of the art, all conventional printing blankets lose thickness (i.e., lose gauge or xe2x80x9csinkxe2x80x9d) when they are initially tensioned and installed, and further lose thickness as the blanket is repeatedly exposed to the interference pressures at the nips between the printing cylinder and blanket-covered cylinder and the blanket-covered cylinder and rigid back-up cylinder, respectively. Blankets can fail catastrophically due to blanket smash, a permanent deformation in a portion of the entire blanket surface, or from a gradual deterioration of blanket gauge over time due to the repeated cycling of the interference pressures on the blanket""s surface. When the thickness of a blanket recedes beyond the limits of press adjustment, the print pressure becomes insufficient to cause transfer of the inked image from the print cylinder to the blanket or the blanket to the paper, or both. Thus, for a typical blanket, a permanent loss of thickness of as little as 0.05 to 0.10 mm (0.002 to 0.004 inches) may require a press stoppage. Such problems are even more severe at or near the gap in the cylinder because there is a tendency of the blanket to xe2x80x9cfall offxe2x80x9d into the gap (i.e., lose the thickness needed to offset the image to the web).
Conventionally, the fixed interference described above is accomplished by inserting one or more thin layers of paper or the like between the blanket and the surface of the blanket cylinder to build up the thickness of the blanket. This process is known as packing a blanket. Once the gauge loss of the blanket reaches a certain amount, as described above, additional thickness must be supplied under the blanket. This involves stopping the press, demounting the blanket and original packing, repacking, and then remounting and retensioning the blanket.
The packing process presents problems however in that the procedure is time consuming, resulting in down time for the printing equipment. Typically, press downtime can cost from several hundred to over a thousand dollars per hour. It may take over 30 minutes to pack or repack a blanket. Further time is lost as the system is retuned to optimum settings. Additionally, once positioned on the cylinder, the packing paper tends to slide, slip, and/or fold which may render the blanket surface nonuniform and result in poor printing results.
To avoid some of the problems associated with packed blankets, some press operators, and in particular news press operators, have used blankets which do not require packing. So-called xe2x80x9cno packxe2x80x9d blankets have been developed to provide a fixed interference without the need to pack the blanket. No pack blankets are manufactured to very precise gauges so that they can be installed directly onto a blanket cylinder with the correct amount of interference. These blankets have the advantage of a one-piece construction which requires no positioning of packing paper beneath the blanket. This results in less down time for the printing equipment when an old blanket is removed and replaced with a new blanket.
Such no pack blankets, like most printing blankets, are normally composed of a base material which gives the blanket dimensional stability. Woven fabrics are preferred. The base typically includes two or more layers of such fabric adhered together. The working surface of the blanket which contacts the ink is typically an elastomeric layer of natural or synthetic rubber which is applied over the base layer or layers. The base layers and working surface are laminated together using suitable adhesives. Again, such blankets exhibit some gauge loss upon initial tensioning and installation and continue to lose thickness over time during use. However, once the gauge (thickness) loss on a no pack blanket exceeds the limits of press adjustment, the blanket becomes unusable without recourse and must be replaced by a new blanket.
An important goal in offset printing is to increase the operating speeds of printing presses in order to maximize production. Typically, conventional flat printing blankets are manufactured so that their ends can be mounted and secured into a relatively wide gap or groove in the blanket cylinder. The gap runs in the axial direction, and the leading and trailing ends of the blanket are inserted into the gap and secured by any of a number of techniques including lock-up mechanisms and clamps. Typically, the leading and trailing ends of the blanket are generally reinforced with strips of metal known as blanket bars to stiffen the blanket ends and facilitate insertion of the blanket into the lock-up mechanism.
However, the need for a gap in the blanket cylinder has resulted in problems when the speed of the cylinder is increased, as the cylinder is unbalanced (i.e., weight is unevenly distributed), and the blanket itself is subjected to increased stresses. This can result in vibrations and shock loading of the blanket, reducing print quality. Newer higher speed presses have appeared which have addressed these problems by providing a smaller gap in the blanket cylinder, sometimes known as xe2x80x9cmini-gapxe2x80x9d presses. Thus, shock loading can be reduced by making the cylinder gap as narrow as possible. Conventional cylinder gap widths, i.e., for use with fabric backed blankets, range from about 5 mm to about 10 mm in width. To address the need for narrow gap blanket cylinders, newer types of printing blankets have been developed. Such blankets are known in the art as metal-backed blankets (see, e.g., International Publication No. WO 93/01003 of Pinkston et al.) which rest upon and are supported by, a thin metal sheet. Metal-backed blankets can be mounted on cylinders with gaps that are less than 3 mm wide. Blanket cylinders having these much-narrower gaps can operate at high speeds with a reduced incidence of shock loading. A further advantage of such narrow gap cylinders is that there is less web area wasted in printing as the print can extend to the narrow gap.
A metal-backed printing blanket typically comprises a base layer of a thin, flat, flexible sheet of metal and a top layer comprising an elastomer such as rubber. Other layers may be sandwiched between the base and top layers, formed of materials such as fabric, after which these multiple layers are laminated together. Such a blanket conventionally has a thickness of about 2 mm, of which about 0.20 mm may be attributed to the thickness of the metal base plate. One configuration of a metal-backed blanket manufactured and sold by KBA (Koenig and Bauer-Albert AG, of Frankenthal, Germany) has a small strip of exposed metal at the leading and trailing edges of the blanket adapted for insertion into the cylinder gap. See, e.g., Puschnerat et al, U.S. Pat. Nos. 5,687,648 and 5,934,194. See also, Castelli et al, U.S. Pat. No. 5,749,298.
Because the thickness of the metal edges is much less than the thickness of the rest of the blanket, the edges may be inserted into a cylinder gap that is much narrower than the gap that is needed to accommodate the thickness of more conventional blankets. However, metal-backed blankets have introduced their own set of problems, including the need for different lock-up mechanisms to avoid blanket pull out during printing operations (i.e., an end of the blanket releases from the lock-up mechanism from the gap). Further, it has not been possible to use these metal-backed blankets on conventional presses because the metal ends will not secure into the conventional lock-up mechanisms found in existing blanket cylinders.
Simply adding a co-extensive metal base layer to a conventional fabric-reinforced printing blanket is not practical as the resulting blanket becomes extremely difficult to mount and tension properly on the blanket cylinder. This is because the added metal, particularly on the leading and trailing ends of the blanket, is relatively inflexible and difficult to feed into the cylinder gap, and the overall blanket is difficult to wrap securely about the blanket cylinder.
Accordingly, there remains a need in the art for an image transfer blanket which resists gauge loss throughout its useful life. Such a blanket would reduce expensive down time for press operators and require fewer adjustments of the press during operation. Further, there remains a need in the art for a such an image transfer blanket which can be retro-fitted onto existing offset presses.
The present invention meets those needs by providing a flexible image transfer blanket which has a non-extensible backing which is easy to mount onto conventional blanket cylinders, which requires no packing (but which can be used with packing), which does not need to be retensioned during operation, and which prints to the gap better than conventional fabric-reinforced blankets. By xe2x80x9cnon-extensiblexe2x80x9d we mean a material which will not elongate under tensions typically used (i.e., typical lock-up mechanisms for image transfer blankets are subjected to a torque force of from between about 2 to about 120 ft-lbs (about 2.7 to about 162 Newton-meters) and apply a tension of from about 100 to about 250 pounds per lineal inch (about 17.8 to about 45 kg/cm)) in the mounting of image transfer blankets. By comparison, typical fabric-reinforced image transfer blankets will elongate by from about 1.25% to about 2.5% of their initial length when subjected to conventional tensioning forces, depending on the particular blanket construction. The image transfer blanket of the present invention is suitable for use with both web-fed and sheet-fed presses.
In accordance with one aspect of the present invention, an image transfer blanket which is adapted to be mounted onto a blanket cylinder is provided and includes first and second ends, with at least one of the first and second ends being adapted to be inserted into the axially-extending gap in the blanket cylinder. The blanket includes an image transfer surface layer, at least one woven fabric ply, and a nonextensible base layer. The at least one woven fabric ply includes both warp and weft fibers, with the weft fibers being oriented so that when the blanket is mounted on the blanket cylinder the weft fibers extend circumferentially about the blanket cylinder. By xe2x80x9cwarpxe2x80x9d fibers, it is meant those fibers which extend lengthwise and which are under tension on a loom or other weaving device. By xe2x80x9cweftxe2x80x9d fibers, it is meant those fibers which are woven around the warp fibers in the fabric. Weft fibers are also sometimes known in the art as pick, fill, or woof fibers. As used herein, the terms fibers and yams are used interchangeably, with fibers referring both to single fibers as well as multiple fiber bundles. By orienting the warp and weft fibers in the fabric ply in this manner, namely so that the weft fibers are oriented around the blanket cylinder when the blanket is mounted, there is sufficient residual elongation in the weft fibers to provide flexibility for bending of the other layers in the blanket in either direction.
Preferably, the blanket includes at least one blanket bar secured to at least one end of the image transfer blanket. Blanket bars are used to secure one or both end of a blanket into the axially-extending gap in the blanket cylinder when the blanket is mounted thereon. A preferred embodiment of the invention includes at least two woven fabric layers in the blanket construction, with at least one of the woven fabric layers, and preferably both of the fabric layers, being oriented such that when the blanket is mounted on the blanket cylinder the weft fibers extend circumferentially about the blanket cylinder.
The base layer is selected from the group consisting of metals and alloys thereof, synthetic polymer resins, and fiber-reinforced synthetic polymer resins. A preferred base layer material comprises steel, polyester, or fiberglass reinforced polymer resin. The blanket construction may also optionally contain a compressible layer.
In accordance with the present invention, the blanket may include a number of features which aid in mounting and securing the blanket on a blanket cylinder. For example, the nonextensible base layer may extend beyond the image transfer surface layer and the woven fabric ply at at least one end thereof. That portion of the nonextensible layer which extends beyond the at least one end of the blanket may be bent such that such portion is adapted to be inserted into the axially-extending gap of the blanket cylinder. That portion of the nonextensible layer may also have a blanket bar secured thereto.
Alternatively, the image transfer surface layer and the woven fabric ply may extend beyond the nonextensible base layer at at least one end thereof. The fabric ply may then be secured within the gap in the blanket cylinder using conventional lock-up mechanisms as are known in this art.
In another embodiment of the invention, a smash-resistant image transfer blanket may be provided and includes an image transfer surface layer, at least one woven fabric ply, and a nonextensible base layer. The at least one woven fabric ply is impregnated with an elastomeric composition and includes warp and weft fibers, the weft fibers being oriented so that when the blanket is mounted on the blanket cylinder the weft fibers extend circumferentially about the blanket cylinder. The elastomeric composition displaces the air in the interstices between the warp and weft fibers to prevent the blanket surface from sinking when subjected to the compressive forces encountered during printing.
Accordingly, it is a feature of the present invention to provide an image transfer blanket which resists gauge loss throughout its useful life. It is a further feature of the invention to provide a blanket which reduces expensive down time for press operators and requires fewer adjustments of the press during operation. It is another feature of the invention to provide a blanket which can be retro-fitted onto existing offset presses. These, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.